A system of controlling ammonia levels in a catalytic exhaust system comprising: means to provide a target value for ammonia slip/ammonia output from said system or a catalytic unit of said system; first comparison means to compare said target value with a feedback value to provide a command value based on said comparison, and means to control the dosing of a reducing agent such as urea into said exhaust system based on said command value; means to input said command value to a transfer function or model to provide an estimated value of ammonia slip/ammonia output from said catalytic unit/system; means to measure actual ammonia slip/ammonia output from said unit/system; second comparison means to compare said actual value with said estimated value; means to provide said feedback value based on the output from said comparison means.